Method of suppressing electromagnetic interference emission

ABSTRACT

An EMI emission suppressing system, apparatus and method that enables the EMI produced by high frequency switching of a switching circuit to be suppressed via the transfer of the higher order harmonic emissions to a frequency range below the standard EMI bandwidth of less than 150 KHz by applying low frequency modulation or jitter into the feedback of a switching signal of the switching circuit. The EMI suppression is achieved with minimal added ripple on the output signal of the switching circuit by using discontinuous modulations in the form of only applying the low frequency modulation when the switch or higher order harmonic producing element of the switching circuit is accessing, or drawing power from, the main power supply.

RELATED APPLICATIONS

This Patent Application claims priority under 35 U.S.C. 119 (e) of theco-pending U.S. provisional application, Ser. No. 61/658,245, filed,Jun. 11, 2012, and entitled “Electromagnetic Interference EmissionSuppressor”. This application incorporates U.S. provisional application,serial number in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates to the field of power supplies. Moreparticularly, the present invention relates to a power converter systemwith modulated control.

BACKGROUND

Every electrical device that connects with a power supply main isrequired not to pollute or transmit high frequency noise onto the main.The amount of electrical emission allowed by electrical devices isheavily regulated by the Federal Communications Commission (FCC).Conventional power supply designs have migrated to using higheroperating frequencies since the higher operating frequencies allow areduction in size of power supply components and allow a reduction incost. The disadvantage of operating at higher frequencies is theincreased production of higher order harmonics or electromagneticinterference (EMI).

Conventional methods of reducing EMI have been aimed at reducing aswitching frequency of a switching circuit below standard EMI bandwidthlimits of 150 KHz as set by the FCC. This approach has the disadvantageof increasing the size of magnetic components in a power supply. Othermethods of reducing EMI have simply been to add additional filtercomponents to reduce unwanted frequency harmonics. This second approachhas the disadvantage of adding to the weight, size and cost of powersupplies. Another approach to reducing large spikes of harmonics or EMIis the use of a snubber circuit. The snubber circuit although effectivein reducing EMI, compromises efficiency of a power converter. In yetanother approach, EMI is reduced by using jitter that takes a discreteharmonic spectrum and spreads the EMI over a continuous frequency range.Conventional systems use jitter by injecting noise into a gate drive orcontroller of the converter. Injecting noise into the gate drive of theconverter has the disadvantage of distorting the output voltage signal.Further, injecting noise directly into a gate drive only applies jitterto the rising and falling edges of the converter switching signal.Moreover, because the jitter is continuous throughout the cycles of theswitching signal, is outside the bandwidth of the amplifier and isinjected outside the regulation loop of the switching circuit, it causesunwanted high ripple on the output of the switching circuit.Accordingly, by continuously injecting jitter directly into the gatedrive circuit, conventional power converters inhibit the efficiency offeedback loop and other features including zero voltage switching andsampling of the switching signal.

SUMMARY OF THE INVENTION

An EMI emission suppressing system, apparatus and method includes afrequency modulation element that modulates the frequency of a switchingelement when the switching element is drawing power from the main powersupply in order to reduce EMI emissions on the main power supply. Thefrequency modulation element discontinues modulation when the switchingelement is not drawing power from the main power supply in order toreduce ripple on the load. As a result, the EMI emission suppressingsystem is able to minimize EMI emissions on the main power supply whilenot outputting excess ripple on the load.

BRIEF DESCRIPTION OF THE DRAWINGS

Several example embodiments are described with reference to thedrawings, wherein like components are provided with like referencenumerals. The example embodiments are intended to illustrate, but not tolimit, the invention. The drawings include the following figures:

FIG. 1 illustrates a functional block diagram of an EMI suppressionsystem according to some embodiments.

FIG. 2 illustrates a circuit diagram of an EMI suppression systemaccording to some embodiments.

FIG. 3 illustrates a flowchart of a method of suppressing EMI emissionaccording to some embodiments.

FIG. 4 illustrates a flow chart of a method of suppressing EMI emissionaccording to an alternative embodiment.

DETAILED DESCRIPTION

Embodiments of the present application are directed to an EMI emissionsuppressing system, device and method. Those of ordinary skill in theart will realize that the following detailed description of the EMIemission suppressing system, device and method is illustrative only andis not intended to be in any way limiting. Other embodiments of the EMIemission suppressing system, device and method will readily suggestthemselves to such skilled persons having the benefit of thisdisclosure.

Reference will now be made in detail to implementations of the EMIemission suppressing system, device and method as illustrated in theaccompanying drawings. The same reference indicators will be usedthroughout the drawings and the following detailed description to referto the same or like parts. In the interest of clarity, not all of theroutine features of the implementations described herein are shown anddescribed. It will, of course, be appreciated that in the development ofany such actual implementation, numerous implementation-specificdecisions must be made in order to achieve the developer's specificgoals, such as compliance with application and business relatedconstraints, and that these specific goals will vary from oneimplementation to another and from one developer to another. Moreover,it will be appreciated that such a development effort might be complexand time-consuming, but would nevertheless be a routine undertaking ofengineering for those of ordinary skill in the art having the benefit ofthis disclosure.

Embodiments of an EMI emission suppressing system, device and method aredescribed herein. The EMI suppressing system, device and method enablesthe EMI produced by high frequency switching of a switching circuit tobe suppressed via the transfer of the higher order harmonic emissions toa frequency range below the standard EMI bandwidth of less than 150 KHz.The transfer of the higher order harmonic emissions is accomplished byapplying low frequency modulation or jitter into the feedback of aswitching signal of the switching circuit. Further, this EMI suppressionis able to be achieved with minimal added ripple on the output signal byusing discontinuous modulations in the form of only applying the lowfrequency modulation when the switch or higher order harmonic producingelement of the switching circuit is accessing or drawing power from themain power supply. As a result, when coupled with the main power supplythe low frequency modulation is applied in order to affect the risingand falling edges of switching pulses of the switching circuit therebysubstantially reducing EMI noise on the main power supply from theswitching circuit. Conversely, when not accessing or drawing power fromthe main power supply the low frequency modulation is suspended in orderto substantially reduce ripple on the output of the switching circuit.

FIG. 1 illustrates a functional block diagram of an EMI suppressingsystem 100 according to some embodiments. As shown in FIG. 1, the system100 includes a power supply 102, a switching element 104, a outputcircuit 106, a feedback element 108 and a frequency modulation element110. The power supply 102 is coupled with the switching element 104which is electrically coupled with the output circuit 106. The feedbackelement 108 is electrically coupled between the load 106 and frequencymodulation element 110 which is electrically coupled with the switchingelement 104. Alternatively, the feedback element 108 is able to bedirectly coupled between the output circuit 106 and the switchingelement 104 and/or the frequency modulation element 110 is able to beelectrically coupled with the switching element 104, the feedbackelement 108 or both.

The power supply 102 is able to include an AC power supply such as amain line or plug outlet. Alternatively, the power supply 102 is able toinclude a DC power supply. The switching element 104 is able to includea power converter circuit, such as a flyback converter. Alternatively,the switching element 104 is able to include other types of circuitsthat include switching elements or otherwise produce EMI emissions. Forexample, the switching element 104 is able to include a forwardconverter, a push-pull converter, a half-bridge converter, a full-bridgeconverter and/or other configurations of switch mode power supplies asare well known in the art. The frequency modulation element 110 is ableto include a signal or clock generator. Alternatively, the frequencymodulation element 110 is able to include other signal generation ormodulation elements that are able to induce low frequency jitter on asignal as are well known in the art. In some embodiments, the frequencymodulation element 110 is able to be integrated with the switchingelement 104, the output circuit 106 and/or the feedback element 108 toform a single integrated circuit. Alternatively, the frequencymodulation element 110 is able to include an individual integratedcircuit that is able to be coupled to one or more separate circuits suchas switching element, output circuit and/or feedback circuits. As aresult, the frequency modulation element 110 is able to provide theadvantage of being coupled to existing integrated circuits in order toimprove their EMI suppression characteristics.

In operation, the switching element 104 periodically couples to or pullspower from the power supply 102 and supplies a desired outputvoltage/signal to a load coupled to the output circuit 106. The feedbackelement 108 detects the output voltage Vout, or a representative valueof the output voltage Vout, and adjusts the output of the switchingelement 104 in order to keep the output voltage Vout within a desiredrange, thereby regulating the output voltage Vout. In some embodiments,the feedback element 108 adjusts the duty cycle of the switching element104 in order to control the output voltage Vout. The frequencymodulation element 110 detects when the switching element 104 is pullingpower from or coupled to the power supply 102 and applies low frequencymodulation or jitter to the signal received from the feedback element108 signal such that EMI caused by the switching element 104 and exposedto the power supply 102 is minimized. The frequency modulation element110 also detects when the switching element 104 is not pulling powerfrom or is not directly coupled to the power supply 102 and ceases ordiscontinues to apply low frequency modulation or jitter on theswitching element signal in order to minimize ripple on the outputvoltage/signal applied to the load. In some embodiments, the frequencymodulation element 110 performs the detection and modulation within thefeedback loop of the switching element 104. As a result, the jitter isonly applied when the switching element 104 is coupled to the powersupply 102 and the system 100 is able to minimize the amount of EMItransmitted on the power supply 102 from the switching element 104 whilealso minimizing the amount of ripple induced on the outputvoltage/signal from the jitter of the frequency modulation element 110.

FIG. 2 illustrates a schematic diagram of an EMI suppression system 200according to some embodiments. The schematic diagram is substantiallysimilar to the functional block diagram shown in FIG. 1 except theadditional details described herein. However, it is understood thatalternative schematics are able to be used to implement the functionalblocks of FIG. 1. As shown in FIG. 2, the EMI suppression system 200includes a power supply 202, a switching element 204, an output circuit206, a feedback element 208 and a frequency modulation element 210. Thesystem 200 is configured to receive an AC voltage signal and to providea regulated DC output voltage Vout that is suitable for many low voltageappliances such as computer laptops, cell phones and other devices. Insome embodiments, the output voltage Vout is able to be within the range5-40V. Alternatively, the output voltage Vout is able to be less than 5Vor more than 40V. In some embodiments, the system 200 is contained on asingle integrated circuit. Alternatively, one or more of the componentsof the suppression system 200 are able to be separate integratedcircuits such that the system 200 is formed by multiple integratedcircuits electrically coupled together.

The power supply 202 includes an AC mains power signal that iselectrically coupled with a rectifier 212 in order to produce anunregulated DC input voltage Vin that is electrically coupled to boththe switching element 204 and the feedback element 208. The outputcircuit 206 includes a diode D1 and capacitor C1. Alternatively, theoutput circuit 206 is able to include an output rectifier circuitcomprising a half or full-wave rectifier. The feedback element 208provides a feedback voltage Vfb, which is representative of the outputvoltage Vout. The frequency modulation element 210 includes the signalgenerator or clock generator 222 and one or more buffers 216. Theswitching element 204 includes a transformer T1, a transistor 218, oneor more resistors R1, R2, R3, a controller device 214, a summationdevice 220 and one or more buffers 216. Alternatively, one or more ofthe summation device 220, the controller device 214 and/or the buffers216 of the switching element 204 are able to be a part of the frequencymodulation element 210 instead of the switching element 204. Indeed, itis understood that one or more of the components of the power supply202, the switching element 204, the output circuit 206, the feedbackelement 208 and/or the frequency modulation element 210 are able to bepositioned or duplicated on one or more of the other elements 202-210.

A first end of the transformer T1 is electrically coupled between theinput voltage Vin received from the power supply 202 and the drainterminal of the transistor 218. The second end of the transformer T1 iselectrically coupled across the diode D1 and capacitor C1 of the outputcircuit 206. The source terminal of the transistor 218 is electricallycoupled with the feedback element 208 and the resistor R3 which iselectrically coupled to ground. The Q output line of the controllerdevice 214 is electrically coupled with the gate terminal of thetransistor 218 and the input of the signal generator 222 of thefrequency modulation element 210 via one or more buffers 216. The Sinput line of the controller device 214 is electrically coupled with theoutput of the summation device 220 via another buffer 216. The inputs ofthe summation device 220 are electrically coupled with the feedbackelement 208, the output of the signal generator 222, and a referencevoltage node Vref that is located between resistors R1 and R2 which areelectrically coupled in series between the input voltage Vin and ground.The summation device 220 can be a comparator, an error amplifier orother device that modulates a difference, or error, between thereference voltage Vref and the feedback voltage Vfb, thereby outputtinga modulated error signal. In some embodiments, the transformer T1 is aflyback transformer.

Alternatively, the transformer T1 is able to be other types oftransformers or load isolating circuitry as are well known in the art.In some embodiments, the signal generator 222 generates a clock signalat a predetermined frequency, such as in the range of 2.0 to 9.0 KHz.Alternatively, the predetermined frequency is able to be less than 2.0KHz, greater than 9.0 KHz, or other predetermined frequencies as arewell known in the art. In some embodiments, the transistor 218 is afield effect transistor such as a n-type metal-oxide-semiconductorfield-effect transistor (MOSFET). Alternatively, the transistor 218 isable to be other types of transistors or switching circuitry as are wellknown in the art. For example, the switching element 204 is able toinclude a variable frequency converter, such that an operating bandwidthof the switching element 204 is able to be adjusted depending on outputpower requirements of the system 200. In some embodiments, thecontroller device 214 is a SR-NOR latch flipflop. Alternatively, thecontroller device 214 is able to be other types of flipflops, pulsewidth modulation circuits or signal logic circuitry able to regulate theduty cycle of the operation of the transistor 218 as are well known inthe art.

In operation, the Q output of the controller device 214 of the switchingelement 204 outputs a switch control signal, or driving signal, to thegate terminal of the transistor 218 that causes the transistor 218 torepeatedly turn on and off. When the switch control signal is high, itactivates the channel of the transistor 218 causing current from thepower supply 202 to be drawn through the primary winding of thetransformer T and the transistor 2181, through the feedback loop 208 andto ground through resistor R3. When the switch control signal is low,the channel of the transistor 218 is deactivated preventing current flowthrough the transistor 218 and therefore the primary winding P1, and thevoltage across the primary winding P1 goes high and power is transferredto the output circuit 206. The summation device 220 receives a feedbacksignal from the feedback element 208 and compares the feedback voltageVfb of the feedback signal to a reference voltage Vref in order togenerate an error signal that is input to the S input of the controllerdevice 214 and adjusts the duty cycle of the switch control signal suchthat a desired output voltage Vout is maintained. As a result, theswitching element 204 regulates the output voltage Vout.

When the switch control signal is high such that the switching element204 is accessing the power supply 202, the signal generator 222 outputsa low frequency modulation or jitter signal to the summation device 220such that the error signal is modulated and the modulated error signalis input to the controller device 214 such that the corresponding switchcontrol signal is modulated according to the low frequencymodulation/jitter signal. As a result, the EMI caused by the switchingelement 204 on the power supply 202 is substantially reduced because themodulation of the switch control signal essentially takes the EMI spikesin the switching frequency and reduces and spreads out the EMI spikesover the frequency spectrum. When the switch control signal is low suchthat the switching element 204 is no longer accessing the power supplyand power stored in the transformer T1 is transferred to the outputcircuit 206, the error signal is no longer modulated with the lowfrequency modulation signal. In some embodiments, the signal generator222 only generates the low frequency modulation signal when theswitching element 204 is accessing the power supply 202. In otherembodiments, the signal generator 222 continuously generates the lowfrequency modulation signal, but the low frequency modulation signal isonly used by the summation device 220 to modulate the error signal whenthe switching element 204 is accessing the power supply 202. As a resultof selectively modulating the error signal and therefore selectivelymodulating the switching control signal, the ripple on the load causedby the modulated switching control signal is also minimized.Accordingly, the EMI emission suppression system 200 provides theadvantage of maintaining maximum reduction of EMI polluted onto thepower supply 202 while also minimizing ripple on the output circuit 206.

FIG. 3 illustrates a flow chart of a method of suppressing EMI emissionaccording to some embodiments. The method steps are described inrelation to the system elements of FIG. 2. It is understood that themethod of suppressing EMI emission can be enabled using alternativesystem element and configurations. At the step 302, a switching element204 selectively accesses, or draws power from, a power supply 202according to a control signal. In some embodiments, accessing the powersupply 202 includes drawing current from the power supply 202. Forexample, accessing the power supply includes activating a transistor 218which is electrically coupled with the power supply 202, thereby drawingcurrent through the primary winding P1 and the transistor 218. In someembodiments, the control signal for activating the transistor 218 is apulse width modulated signal output by a controller device 214. In someembodiments, the control signal is adjusted by an error signal generatedby a summation device 220 which compares a reference voltage Vref with afeedback signal Vfb. At the step 304, a frequency modulation element 210generates a low frequency modulation signal, or jitter, when theswitching element 204 is accessing the power supply 202. In someembodiments, the low frequency modulation signal is generated at apredetermined frequency in the range of 2.0 to 9.0 KHz. Alternatively,the predetermined frequency is able to be less than 2.0 KHz, greaterthan 9.0 KHz, or other predetermined frequencies as are well known inthe art. At the step 306, a modulated control signal of the switchingelement 204 is generated according to the low frequency modulationsignal. The switching element 204 selectively accesses the power supply202 according to the modulated control signal thereby minimizing theamount of EMI of the switching element 204 output on the power supply202. At the step 308, the frequency modulation element 210 discontinuesgenerating the low frequency modulation signal when the switchingelement 204 is not accessing the power supply 202 so that the controlsignal is not modulated according to the low frequency modulationsignal, thereby minimizing the amount of ripple output on the outputcircuit 206, where ripple is caused by the modulation or jitter of themodulated control signal. As a result, the method provides the advantageof enabling EMI emission on the power supply to be minimized while alsominimizing the amount of ripple on the load.

In an alternative embodiment, the frequency modulation element 210continuously generates the low frequency modulation signal, but thecontrol signal is only modulated according to the low frequencymodulation signal when the switching element 204 is accessing the powersupply 202. FIG. 4 illustrates a flow chart of a method of suppressingEMI emission according to this alternative embodiment. At the step 402,the switching element 204 selectively accesses, or draws power from, thepower supply 202 according to a control signal. At the step 404, thefrequency modulation element 210 generates a low frequency modulationsignal, or jitter. At the step 406, the low frequency modulation signalis applied when the switching element accesses the power supply 202,thereby generating a modulated control signal according to the lowfrequency modulation signal. At the step 408, application of the lowfrequency modulation signal is discontinued when the switching element204 is not accessing the power supply 202 so that the control signal isnot modulated according to the low frequency modulation signal.

The method, apparatus and system of EMI emission suppression describedherein has many advantages. Specifically, the system minimizes the EMIemission output on a power supply by a switching element by modulatingthe frequency of the switching element using low frequency modulation ina feedback loop. Further, the system minimizes the ripple output on theload resulting from the low frequency modulation by using discontinuousmodulation where the control signal is only modulated when the switchingelement is accessing the power supply. Thus, the EMI emission suppressordescribed herein has numerous advantages.

The EMI emission suppressing system, device and method is describedabove in terms of modulating a control signal used to regulate an outputvoltage. It is understood that the EMI emission suppressing system,device and method can alternatively be applied to non-regulatingapplications. Such alternative applications may or may not include afeedback element. The modulating signal may be applied to a reference orother signal used by the switching element to generate the modulatedcontrol signal.

The present application has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the EMI emission suppressingsystem, device and method. Many of the components shown and described inthe various figures can be interchanged to achieve the resultsnecessary, and this description should be read to encompass suchinterchange as well. As such, references herein to specific embodimentsand details thereof are not intended to limit the scope of the claimsappended hereto. It will be apparent to those skilled in the art thatmodifications can be made to the embodiments chosen for illustrationwithout departing from the spirit and scope of the application.

What is claimed is:
 1. A method of regulating a power converter having aswitching circuit, the method comprising: a. selectively drawing powerfrom a power supply according to a control signal used to drive theswitching circuit; b. generating a low frequency modulation signal whenthe switching circuit draws power from the power supply; and c. applyingthe low frequency modulation signal so as to generate a modulatedcontrol signal that is modulated according to the low frequencymodulation signal.
 2. The method of claim 1 wherein the modulatedcontrol signal substantially reduces higher order harmonic emissionscorresponding to switching of the switching element.
 3. The method ofclaim 1 wherein the low frequency modulation signal is generated at apredetermined frequency.
 4. The method of claim 3 wherein thepredetermined frequency comprises a frequency in the range of 2 to 9Khz.
 5. The method of claim 1 applying the low frequency modulationsignal comprises: a. generating a modulated error signal that ismodulated according to the low frequency modulation signal; and b.supplying the modulated error signal to a controller circuit of theswitching circuit, wherein the controller circuit generates themodulated control signal.
 6. The method of claim 1 wherein the controlsignal comprises a pulse width modulated signal for controlling a dutycycle of the switching circuit.
 7. The method of claim 1 furthercomprising regulating an output voltage of the power converter.
 8. Themethod of claim 7 further comprising supplying a feedback signal to theswitching circuit, wherein the feedback signal is representative of theoutput voltage.
 9. The method of claim 1 further comprisingdiscontinuing generation of the low frequency modulation signal when theswitching circuit is not drawing power from the power supply.
 10. Themethod of claim 1 wherein driving the switching circuit with themodulated control signal substantially reduces harmonic distribution ina switching frequency of the switching circuit.
 11. A method ofregulating a power converter having a switching circuit, the methodcomprising: a. selectively drawing power from a power supply accordingto a control signal used to drive the switching circuit; b. generating alow frequency modulation signal; and c. applying the low frequencymodulation signal when the switching circuit draws power from the powersupply so as to generate a modulated control signal that is modulatedaccording to the low frequency modulation signal.
 12. The method ofclaim 11 wherein the modulated control signal substantially reduceshigher order harmonic emissions corresponding to switching of theswitching element.
 13. The method of claim 11 wherein the low frequencymodulation signal is generated at a predetermined frequency.
 14. Themethod of claim 13 wherein the predetermined frequency comprises afrequency in the range of 2 to 9 Khz.
 15. The method of claim 11applying the low frequency modulation signal comprises: a. generating amodulated error signal that is modulated according to the low frequencymodulation signal; and b. supplying the modulated error signal to acontroller circuit of the switching circuit, wherein the controllercircuit generates the modulated control signal.
 16. The method of claim11 wherein the control signal comprises a pulse width modulated signalfor controlling a duty cycle of the switching circuit.
 17. The method ofclaim 11 further comprising regulating an output voltage of the powerconverter.
 18. The method of claim 17 further comprising supplying afeedback signal to the switching circuit, wherein the feedback signal isrepresentative of the output voltage.
 19. The method of claim 11 furthercomprising discontinuing application of the low frequency modulationsignal when the switching circuit is not drawing power from the powersupply.
 20. The method of claim 11 wherein driving the switching circuitwith the modulated control signal substantially reduces harmonicdistribution in a switching frequency of the switching circuit.
 21. Amethod of regulating a power converter having a switching circuit, themethod comprising: a. injecting a low frequency modulation signal into asummation circuit configured to output an error signal; b. modulatingthe error signal of the summation circuit using the low frequencymodulation signal when the switching circuit is drawing power from apower supply; and c. using the modulated error signal to substantiallyreduce harmonic distribution in a switching frequency of the switchingcircuit.
 22. The method of claim 21 further comprising generating thelow frequency modulation signal at a predetermined frequency using asignal generator.
 23. The method of claim 22 wherein the predeterminedfrequency comprises a frequency in the range of 2 to 9 Khz.
 24. Themethod of claim 21, further comprising: a. applying the modulated errorsignal to a controller circuit for the switching circuit; and b.generating a pulse width modulated signal for controlling a duty cycleof the switching circuit.
 25. The method of claim 21 further comprisingregulating an output voltage of the power converter.
 26. The method ofclaim 25 further comprising supplying a feedback signal to the summationcircuit, wherein the feedback signal is representative of the outputvoltage.
 27. The method of claim 21 further comprising discontinuingmodulation of the error signal using the low frequency modulation signalwhen the switching circuit is not drawing power from the power supply.